In recent years, a machining method using a laser beam has been applied, in processes of manufacturing products and components or the like, for applications difficult with the conventional machining method. Especially in a manufacturing process of printed circuit boards, the tendency for minimization and higher integration degree of a circuit for electronic equipment has been becoming increasingly visible, and it is required also in a boring process executed in a process of manufacturing printed circuit boards to bore fine holes which are so fine that the conventional drilling technology can not be applied to. As an alternative for the boring process with a drill, a boring process with a laser is applied for boring a printed circuit board.
When boring a printed circuit board with a laser, a pulse-formed laser beam emitted from a laser oscillator is converged onto a particular point on the printed circuit board, and a substrate material made of epoxy resin or the like on the irradiated portion is removed by means of thermal decomposition. It is generally required to machine thousands to tens of thousands of holes per substrate for a comparatively high-density printed circuit board which requires machining of fine holes with a laser beam as described above, and in order to bore such a large number of holes within a short period of time, a position to which a laser beam is to be irradiated should rapidly be moved, and a galvanometer scanner is used for that purpose as a high-speed positioning unit for a laser beam.
For example, FIG. 4 is a view for explaining general configuration of a laser machining apparatus 50 for a process of boring a printed circuit board based on the conventional technology using a galvanometer scanner. One of laser machining apparatuses based on the conventional technology using a galvanometer scanner is disclosed in Japanese Patent Laid-Open Publication No. HEI 5-2146, and especially as a laser machining apparatus for a process of boring a printed circuit board, there is the one disclosed in Japanese Patent Laid-Open Publication No. HEI 7-32183.
Description is made hereinafter for operations of the laser machining apparatus 50 for a process of boring a printed circuit board based on the conventional technology with reference to FIG. 4. At first, a pulse-formed laser beam LB emitted from a laser oscillator 52 is reflected by a pair of rectangular deflecting mirrors 54, 56 each with abase material of the mirror made of fused quartz, the reflected laser beam LB is converged through a f.theta.lens 64 and irradiated onto a printed circuit board 66. Each of the deflecting mirrors 54, 56 is fixed to each end of shafts 68 for a pair of galvanometer scanners 58, 60 each fixed to a scanner holding section 62 respectively.
The galvanometer scanners 58, 60 are connected to a scanner driving amplifier 85. This scanner driving amplifier 85 rotates, by sending to the galvanometer scanners 58, 60 a current instruction corresponding to a positional instruction issued from an instructing section 88 in synchronism with each pulse of a laser beam, each of the shafts 68 by an angle corresponding to the positional instruction, swings the deflecting mirrors 54, 56, and positions the laser beam LB for each pulse at a particular position on the printed circuit board 66, so that a boring process is executed.
In this process, since an area on the printed circuit board 66 onto which a laser beam can be irradiated through the f.theta.lens 64 is only a finite scanned area 67, the printed circuit board 66 is successively moved within a plane vertical to the laser beam with an XY table 69 driven by an NC unit 89 each time when machining for one scanned area 67 is finished so that all the scanned areas 67 on the board can be machined.
Next description is made for the structure and operational principles of the galvanometer scanners 58 and 60. The galvanometer scanner is an electric motor of which shaft can rotate only in a finite angular range, and is also a drive unit for swinging a deflecting mirror to deflect a laser beam which is generally used in any optical device using a laser beam.
FIG. 5 shows a block diagram of the galvanometer scanner 58 as well as of a control circuit for the scanner. The galvanometer scanner 60 has the configuration as shown in FIG. 5, although not shown in the figure. In FIG. 5, the galvanometer scanner 58 comprises a shaft 68 for swinging the deflecting mirror 54, a permanent magnet 70, a drive coil 76, and a static capacitance type of angle detector 80 fixed to the shaft 68 for detecting angular displacement of the shaft 68.
A biased magnetic field 74 generated by the permanent magnet 70 and a driving magnetic field 78 generated by passing a current through the drive coil 76 are indicated by arrows in the figure, and rotating torque to the shaft 68 is generated by interaction between the biased magnetic field 74 and the driven magnetic field 78.
The scanner driving amplifier 85 as a control circuit for the galvanometer scanner 58 can provide controls so as to rotate the shaft 68 in a specified direction by a specified angle by amplifying a displacement error signal as a difference between an angular displacement signal outputted from the static capacitance type angle detector 80 and a positional instruction as an input signal by a current amplifier 84 and changing the amplified signal to a driving current for being passed to the drive coil 76. Namely, servo control is executed by feeding back a displacement signal outputted from the static capacitance type angle detector 80 to an input signal and constituting a closed loop.
The galvanometer scanner 58 has the configuration as described above, the deflecting mirror 54 is fixed to the end of the shaft 68, and the pair of galvanometer scanners 58, 60 are placed as shown in FIG. 4, so that a laser beam can be irradiated onto a specified position on the printed circuit board 66 according to a positional instruction from the instructing section 88.
FIG. 7 shows a perspective appearance view of the galvanometer scanner 58. The galvanometer scanner 58 has a cubic form here, and screw holes 86 for fixing the main body of the galvanometer scanner 58 are provided on one face of the cube. In the laser machining apparatus for a process of boring a printed circuit board based on the conventional technology using the galvanometer scanner 58, the galvanometer scanner 58 is fixed to the scanner holding section 62 through the screw holes 86 provided on the side face of the main body of the galvanometer scanner without giving any rotational flexibility to the shaft 68 to the main body of the galvanometer scanner as shown in FIG. 4.
The deflecting mirror 54 used in the laser machining apparatus for a process of boring a printed circuit board based on the conventional technology using the galvanometer scanner employs a mirror substrate made of fused quartz having comparatively low solidity, and is a rectangle as shown in FIG. 8.
However, with the laser machining apparatus for a process of boring a printed circuit board using the galvanometer scanner based on the conventional technology as described above, when a deflecting mirror is fast swung for positioning a laser beam within a short period of time, uncontrollable vibrations occur on the shaft 68 due to unbalance between rotational centers which the shaft 68 of the galvanometer scanner 58 and the deflecting mirror 54 have respectively, and deflection occurs in the deflecting mirror 54 fixed to the end of the shaft 68, so that a laser beam can not accurately be positioned to a specified position on the printed circuit board, which causes positional accuracy of machined holes to be reduced.
Especially, reduction of the positional accuracy is significant when extremely high-speed positioning is performed for 500 points or more per sec, which becomes a great obstacle in machining a high integration degree printed circuit board requiring a large number of holes to be machined with high accuracy within a short period of time.
Description is made herein for the unbalance between a rotational center of the shaft 68 of the galvanometer scanner 58 and that of the deflecting mirror 54 which causes the reduction of positional accuracy. The shaft 68 of the galvanometer scanner 58 and the deflecting mirror 54 fixed to the end of the shaft 68 have eccentricities .epsilon.1, .epsilon.2 each against the rotational center respectively.
Those eccentricities .epsilon.1, .epsilon.2 are generated by an error in manufacturing the galvanometer scanner 58 and the deflecting mirror 54, and it is difficult to manufacture the shaft 68 and the deflecting mirror 54 each with complete symmetry with respect to rotation for the purpose to eliminate the eccentricity. The unbalance is expressed by a product of mass and eccentricity, and unbalance U1 of the deflecting mirror 54 is expressed as follows: EQU U1=m1.epsilon.1
On the other hand, unbalance U2 of the shaft 68 is expressed as follows: EQU U2=m2.epsilon.2
wherein, m1 indicates mass of the deflecting mirror 54, and m2 indicates mass of the shaft 68.
By the way, unbalance U synthesized when the deflecting mirror 54 is fixed to the end of the shaft 68 so that both of the components are made to one-piece is expressed as follows: EQU U=U1+U2
As shown in FIGS. 9A and 9B, when an angle .theta. of U2 to U1 is used, the expression is changed as follows: EQU U=U1+U2 cos.theta.
Herein, as shown in FIG. 9A and FIG. 9B, the following conditions are satisfied: EQU when .theta.&lt;90 degrees and 270 degrees&lt;.theta., U2 cos.theta.&gt;0 EQU when 90 degrees&lt;.theta.&lt;270 degrees, U2 cos.theta.&lt;0
and the synthesized unbalance U changes in degrees according to a positional relation between the eccentricities against the rotational center, namely, according to a mounting angular position of the deflecting mirror 54 to the shaft 68, so that, when the degree is set to 180 degrees: .theta.=180 degrees, the synthesized unbalance can be made to a minimum value.
In a rotating body, if a rotational speed of a shaft is higher, an allowable value to the unbalance which causes vibrations is generally smaller. The above fact is also effected in a case where a galvanometer scanner is used, and it is required to make smaller the unbalance synthesized when the shaft 68 and the deflecting mirror 54 are made to one-piece for the purpose of rapidly rotating the shaft 68 so that a laser beam can more rapidly be positioned.
Description is made hereinafter for a difference between operations of the galvanometer scanner according to difference in degrees of unbalance. FIGS. 6A to 6C are time charts showing synchronism between irradiation of a laser beam and operations of the galvanometer scanner in a laser machining apparatus for a process of boring a printed circuit board using a galvanometer scanner.
In the chart, FIG. 6A shows a trigger signal for emission of a laser beam, FIG. 6B shows an input signal indicating a positional instruction to a galvanometer scanner, and FIG. 6C shows a displacement error signal indicating a difference between a positional instruction and displacement generated by rotating a deflecting mirror according to the instruction. A shaft of the galvanometer scanner is rotated in an area where a positional instruction is being changed, and displacement of the shaft is gradually converged toward the targeted position in a stabilized area where the positional instruction is constant, so that a displacement error signal is reduced to zero. At this point of time when this displacement error signal is zero, a laser beam is emitted in pulses according to a trigger signal for laser emission, and by irradiating the laser beam onto a printed circuit board, a hole is made at an accurate position.
When a mounting angular position of the deflecting mirror 54 to the shaft 68 is appropriate and unbalance is suppressed to a minimum, the displacement error signal is smoothly converging to zero as indicated by a solid line in the stabilized area, and in contrast to the above case, when the mounting angular position of the deflecting mirror 54 to the shaft 68 is inappropriate and large unbalance occurs, the displacement error signal is vibrated due to vibrations of the shaft 68 as indicated by a broken line, and operational performance of the galvanometer scanner are resultantly reduced, so that there occurs displacement between a position to which a laser beam is irradiated and an accurate position on the printed circuit board because the deflected mirror can not be stopped at the accurate position.
However, in the laser machining apparatus for a process of boring a printed circuit board based on the conventional technology using a galvanometer scanner, as shown in FIG. 4, each of the galvanometer scanners 58, 60 is fixed to the scanner holding section 62 without giving any rotational flexibility to the shaft 68 to the main body of galvanometer scanner.
For this reason, when the positional instruction to the galvanometer scanners 58, 60 is zero, namely when the shaft 68 is positioned at a midpoint of the maximum rotational angle, to position a laser beam at a center of a scanned area 67, each mounting angular position of the deflecting mirrors 54, 56 to each of the shafts 68 of the galvanometer scanners 58, 60 has to be decided unitarily.
As described above, once each mounting angular position of the deflecting mirrors 54, 56 to each of the shafts 68 of the galvanometer scanners 58, 60 is decided unitrarily, each of the deflecting mirrors 54, 56 can not be fixed to a mounting angular position to the shaft 68 where unbalance synthesized between the shaft and the deflecting mirror is minimized as described above, and for this reason it is difficult to suppress the unbalance to the minimum level.
As a result, when a high-speed positioning operation is carried out, the operational performance of the galvanometer scanner are extremely reduced, and the deflecting mirror 54 is deflected due to generation of vibrations when the shaft 68 is stabilized at the moment at which a laser beam is irradiated, so that positioning accuracy for a laser beam is reduced. As described above, when the galvanometer scanners are held, unbalance in the shaft can not be suppressed, so that it is difficult to bore the printed circuit board 66 with high speed and high accuracy.
On the other hand, when a deflecting mirror is rapidly swung for positioning a laser beam with high speed and high accuracy, it is important to make a moment of inertia that the deflecting mirror has as small as possible.
However, when a laser beam LB being circular in cross section is reflected as shown in FIG. 8 on a rectangular deflecting mirror comprising a mirror substrate made of fused quartz crystal based on the conventional technology, there is extra area 55 which is not dedicated to reception of the laser beam LB, so that the moment of inertia is large, and as a result, the deflecting mirror 54 can not be swung at a high speed by the galvanometer scanner.
Especially, the fused quartz crystal used for the conventional type of deflecting mirror has comparatively low solidity, so that the mirror substrate is easily distorted, and it is difficult to manufacture a thin mirror substrate, and as a result the mirror substrate can not be made thinner, which makes it difficult to suppress the moment of inertia to a low level.